The relationship between Toxoplasma gondii and cerebritis in AIDS patients was studied. Toxoplasma was found to be the most frequently recognized cause of cerebral mass lesions in AIDS. Toxoplasma was demonstrated in 7 of 22 (37%) of AIDS patients having an anti-toxoplasma titer equal or greater than 1:256. Specific antigenic determinants on the surface of Toxoplasma cystozoites appear to be lost after a brief period (3 days) of multiplication within skeletal muscle cell cultures. Preliminary experiments suggest that ELISA techniques can be used to detect circulating anodic antigen from patients with schistosomiasis. The Dot-ELISA and the ELISA were compared with the indirect fluorescent antibody test (IFAT) for detection of IgM and IgG specific antibodies to Toxoplasma. The Dot-ELISA correlated well with the IFAT (corr. coef. 0.895) and the ELISA correlated equally well with the IFAT (corr. coef. 0.910) for detecting IgG antibodies to T. gondii.